Modified rate equations revisited. A corrected treatment for diffusive reactions on grain surfaces

It is well known that the rate equations for diffusive reactions on grain s urfaces can be inappropriate under certain circumstances because they do no t take the discrete nature of grains into account. A previous modification of the rate equations developed by us to treat surface chemistry on grains more accurately contains an error in the probability of evaporation from gr ain surfaces. With the rate of evaporation handled correctly, we show for a simple system in which only O and H atoms accrete on grain surfaces and re act to form H-2, OH,and O-2 that the modified rate method is in reasonable agreement with a corrected Monte Carlo approach through 20 K and in excelle nt agreement with a new master equation approach at 10 K.